If I was a water bender and had to kill someone evil, I'd do it as follows:

Shoot ice arrows up through the ground/persons feet that would hook them to the ground. Then I would command an ice spire to shoot up in-between the persons legs and stop center-mass. Afterwards, I would cause the ice to "snowflake" out of the persons body.

Awesome lol Something cool about water bending is how creative/versatile it can be. You could boil the person, freeze them, dehydrate them, even make them combust if you increased the temperature by taking away the cold. The possibilities are limitless.

hate to be that guy, but... cold is just the absence of heat energy, I dig your thought process but i think only firebenders have the power to add that kind of energy for combustion/boiling of fluids. Unless I missed some lore, in which case i shall gladly submit to seppuku.

It's safe to assume Water benders can reduce/increase kinetic energy (make liquids freeze/boil) Combustion is different though I guess. I see your point. But they could at least make it much easier for certain things to catch on fire lol. They could super-dehydrate a field of wheat, then use the steam to make it very hot in one area and that MIGHT result in a fire?